Optimization of Cr4+-doped saturable-absorber Q-switched lasers

Cr⁴⁺-doped saturable absorbers are characterized by long excited state lifetime and appreciable excited state absorption. In the paper, we first solve the three coupled rate equations describing the operation of Cr⁴⁺-doped saturable-absorber passively Q-switched lasers to obtain the expressions of pulse characteristics such as output energy, peak power, and pulsewidth. We then determine the key parameters of an optimally coupled passively Q-switched laser as functions of two variables concerning the amplifying medium, saturable-absorber medium, and pump level, and generate several design curves. These key parameters include the optimal normalized coupling parameter and the optimal normalized saturable-absorber parameter which maximize the output energy (or maximize the peak power, or minimize the pulsewidth), and the corresponding normalized energy, normalized peak power, and normalized pulsewidth. The results are valid for not only Cr ⁴⁺-doped saturable-absorber Q-switched lasers but also any other lasers passively Q-switched by saturable absorbers with long excited state lifetime and appreciable excited state absorption. Using the expressions and design curves, with the aid of a simple hand calculator, one can predict the pulse characteristics and perform the design of an optimally coupled passively Q-switched laser